Flexible pipes for transportation of hydrocarbons are well known in the art. In this text the term “unbonded” means that at least two of the layers of the pipe are not bonded to each other. Often unbonded flexible pipes comprise an internal sheath (often called an inner liner) which forms a barrier against the outflow of the fluid which is conveyed through the pipe, and one or more armoring layers on the outer side of the internal sheath (outer armoring layer(s)). The flexible pipe may comprise additional layers such as a carcass which is an internal armoring layer to prevent the collapse of the internal sheath. An outer sheath may be provided with the object of forming a barrier against the ingress of fluids from the pipe surroundings to the armoring layers. Often the pipe will comprise at least two armoring layers which are not bonded to each other directly or indirectly via other layers along the pipe. Thereby the pipe becomes bendable and sufficiently flexible to roll up for transportation.
The above-mentioned type of flexible pipes is used, among other things, for off-shore as well as some on-shore applications for the transport of fluids and gases. Flexible pipes can e.g. be used for the transportation of fluids where very high or varying pressures exist along the longitudinal axis of the pipe, such as riser pipes which extend from the seabed up to an installation on or near the surface of the sea, pipes for transportation of liquid and gases between installations, pipes which are located at great depths on the seabed, or pipes between installations near the surface of the sea.
A problem often observed when transporting hydrocarbons is the formation of hydrates, in particular hydrates of relatively small hydrocarbons, such as methane. Also gases such as CO2 and H2S may form hydrates. These hydrates are often referred to as clathrate hydrates. The formation of clathrate hydrates takes place at suitable temperatures and pressures, depending on the fluid transported. Clathrate hydrates are not chemical compounds as the sequestered molecules are never bonded to the lattice. Their detailed formation and decomposition mechanisms on a molecular level are still not well understood. The clathrate hydrates may deposit within the pipe and form plugs. It has been found that by keeping the pressure within the pipe relatively low, the formation of hydrates may be reduced, however, simultaneously the hydrocarbon flow and thereby the amount of fluid pumped through the pipe will be reduced accordingly.
In U.S. Pat. No. 5,625,178 and U.S. Pat. No. 5,998,681 are described processes for preventing the formation of hydrates by subjecting the fluid to pulses of electromagnetic waves.
In U.S. Pat. No. 7,279,052 is described a method of removing deposited hydrates from a pipeline by inserting a thrusters pig.
EP 2 065 551 describes a flexible pipe with two or more optical fiber strain sensors incorporated in the internal sheath or helically wound around the internal sheath. Based on these fiber strain sensors a pressure in the bore of the pipe can—in according to EP 2 065 551—be calculated.
A flexible pipe of the type in question has an elastic working interval where the forces pulling in the length direction are in balance (pulling forces) with the pressure difference between the inside of the pipe (the bore) and the pressure acting in the outer side of the pipe (delta bore pressure). In this elastic working interval the pipe will be elongated when the pulling forces exceed the delta bore pressure and visa verse—such variations may be detected by the optical fiber strain sensors of EP 2 065 551 and are generally measured by measuring the hoop stress. However, since a flexible pipe in use will be subjected to a plurality of different and varying forces which will cause the pipe to move and flex, the optical fiber strain sensors will pick up large amounts of signal variations which are unrelated to changes in bore pressure and need to be filtered out subsequently. Though the optical fiber strain sensors may provide measurements which can detect local formations of blockages, it is very difficult to determine the actual pressure within the bore to a sufficient degree of accuracy to avoid formation of hydrate. Furthermore, a large number of fiber strain sensors arranged in different pitches will be required along the entire pipe length.
The object of the invention is to provide an unbonded pipe by use of which it is possible to optimize high hydrocarbon flow while minimizing or eliminating hydrate formation. This object has been achieved by the invention as claimed. Other benefits and improvements of embodiments of the invention will be clear to a skilled person from the description and example given below.